Circuits that combine optoelectronic (OE) and electronic devices have applications in communication (e.g. as transmitters and receivers for optical fiber based links or for free-space interconnections). Such circuits also are used to form optoelectronic switched networks in applications such as photonic beam forming for phased-array antennas. Switched networks are also used for multi-sensor and multi-processor systems, multi-channel radar imaging, high speed data routing, and digital beam forming.
Commonly, circuits that combine OE and electronic devices are formed of a number of separate components manufactured separately and combined in a hybrid-packaged assembly. Such assemblies have the deficiencies of degraded performance due to packaging parasites. In addition, there is additional cost that results from the hybrid assembly procedures. Monolithic circuits, in which the OE and electronic devices are formed on a common substrate, have been developed as an alternate to the hybrid packaging and assemblies.
One known monolithic optoelectronic receiver is based on a standard, single-heterojunction bipolar transistor (HBT). In this approach, the collector of the standard HBT also serves as the light-absorbing layer of the photodetector. The base, collector and subcollector layers are all fabricated from the smallest bandgap material of the overall structure. This results in these layers being optically absorbing. In the known example, InGaAs is used. One problem with this approach is that only receivers and not transmitters, modulators or optoelectronic switches may be fabricated. This is due to the unattenable characteristics of the optically absorbing layers.
Other known devices have optoelectronic modulators and detectors that have been pseudo-monolithically integrated with transistors. These devices are heterostructure field effect transistors. The modulator and photodetector of such devices have multiple-quantum well structures. The only epitaxial layer that the electronic and optoelectronic devices have in common is the top layer and, thus, are only minimally monolithically integrated. One problem with such devices is that the devices have low current driving capabilities. Another drawback of such devices is that high-resolution photolithography is used to form the gate structure. High resolution photolithography is an expensive process.
Another known heterojunction phototransistor uses a light absorbing layer that is part of the collector. The light absorbing layer has a smaller bandgap than the emitter, base and subcollector layers. The light absorbing layer is intentionally kept thin so as not to degrade the performance of the transistor. One problem with such device is that because the light absorbing layer is so thin, a multi-layer reflector underneath the subcollector is needed to create a resonant optical cavity to enhance the photo sensitivity.